All Perfect Praise be to Allaah, the Lord of the Worlds; and may He exalt the mention of Prophet Muhammad as well as all his Family and Companions.
Basically the funeral prayer should be offered on a dead person whilst placing his corpse in front of those who pray on him. As long as the search of the corpse is going on, and getting it is possible, it is better to wait until you find it.
However, if it is strongly believed that your friend has died and you cannot find her corpse, then there is no harm – Allaah willing – to perform the absentee funeral prayer on her.
Allaah knows best.